In drier loops, for example, in plants for sludge drying, granulate with different grain sizes is produced from the material being dried. The size of the dried grains depends on the operating status of the plant in each case and on the nature of the material being dried, including, for example, the composition, moisture content and the condition of the sludge. These different grain sizes are usually divided into separate fractions using a filter screen, A roller mill is used to crush the large and some of the medium fraction of the dried particles in order to provide the desired amount of fine material required for back-feeding to the incoming fresh material to be dried. The dried fraction is often mixed with fresh wet material to increase the volume of the material and reduce the ratio of the water to solids. The roller mill, however, has the disadvantage of high investment costs and high energy consumption. In addition, it is subject to a great deal of wear and, as a consequence, requires extensive servicing, thus generating high maintenance costs.
Accordingly, there is a continuing need in the industry for a drying process that is more efficient.